A type of interference caused by passive intermodulation products (PIM) occurs in high power radio frequency (RF) communications systems which involve simultaneous transmit and receive operation of RF energy. The problem of PIM generation is particularly acute where the transmit and receive frequencies are closely spaced, which is typically the case for high performance satellite-based systems due to the limited availability of frequency spectrum allocated for a specified communication. The passive metal-to-metal junctions of such satellite-based systems, and in particular, the loose metal junctions of the RF reflective mesh antennae of such satellite-based systems, have been identified as sources of passive intermodulation products (PIM).
The generation of intermodulation products in passive metal-to-metal junctions arises because most metals in air intrinsically posses a thin layer of oxidation which will act as an insulator. When two metal bodies are loosely joined, a metal-insulator-metal interface is produced. Before contact, the insulator acts as a dielectric. As the metal bodies are brought into contact or near contact under pressure, the oxidation layer functions as a semiconductor. Such passive metal-insulator-metal interfaces exhibit nonlinear behavior which will produce interference signals (PIM) that are generated by the combination of harmonics of two adjacent transmit frequencies. When this signal interference or PIM falls within the receive band frequency, system performance is compromised.
Accordingly, the design of a high performance PIM free mesh antenna, and particularly a deployable PIM free mesh antenna, must meet several requirements to ensure against the formation of loose metal-to-metal contacts which will cause PIM. First, the mesh material, in addition to being a good conductor of RF energy, should also be flexible and mechanically pliable to permit wrinkle-free packaging. At the same time, the mesh material should be sufficiently robust such that the individual conductive strands or wires will not kink when folded during stowage or break when loaded in tension during deployment. Further, in order to maintain an accurate reflector surface configuration, the mesh material should also be stretch resistant to prevent sagging.
Prior art mesh antennae are typically constructed from woven synthetic yarns which are spray painted with conductive metallic paint, such as copper paint. It is also known from the prior art how to construct a mesh antenna using thin gage metal wires which are knitted together to form a tricot mesh. Such prior art mesh antenna designs, however, typically exhibit undesirables high in-plane mechanical stiffness and are prone to wrinkling. Further, such designs inherently cause PIM, since the continuity of the metallic coating (in the case of a metal spray coated woven yarns) or the continuity of the metal-to-metal junctions (in the case of knitted metal wires) is difficult to maintain because of the mechanical handling involved in the packaging and the deployment of the reflector.
In the case of a parabolic or dish shaped reflector having a reflective antenna surface composed of a plurality of gore shaped mesh panels, care must be taken when cutting the electrically conductive mesh material into the individual gore shapes in order to avoid the possible generation of PIM sites in the resulting reflective antenna surface. In the usual practice, the mesh material is simply cut along a particular line of geometry without attention to which conductors of the mesh material and in which fashion they are cut. However, in order to prevent the formation of loose metal-to-metal contacts and sparking from occurring between the conductors of the mesh material, it is necessary to ensure that the exposed cut ends of the conductors are maintained at a certain minimum distance spacing from each other.
Further, care must be taken to avoid damage to the individual metal wires or conductive strands of the mesh material when attaching the mesh material to the support ribs of the reflector. In accordance with the conventional practice, the mesh material is attached by sewing the mesh material directly to the support ribs of the antenna structure. This attachment technique results in localized high stress at the points of attachment between the mesh material and the ribs. This will often lead to kinking and/or breakage of individual knitted wires or conductive strands thereby resulting in the formation of loose metal-to-metal contacts and hence PIM.
Accordingly, there is a definite need in the art for an improved electrically conductive mesh material for use as a high performance RF reflective antenna surface which overcomes the problems of the prior art and which by itself does not produce PIM.
Further, there is a need in the art for a method of cutting electrically conductive mesh material into a desired reflector panel shape such that the exposed cut ends of the mesh material are prevented from coming into contact or near contact with other in order to inhibit sparking and generation of PIM.
Further still, there is a need in the art for a method of attaching an electrically conductive mesh material to its supporting structure in such a manner which distributes the stress on the mesh material in the region of attachment over a broader surface area so as to avoid kinking or breakage of individual wires or conductive strands of the mesh material.